[#1502] Use zap.Error for logging errors

Use `zap.Error` instead of `zap.String` for logging errors: change all expressions like
`zap.String("error", err.Error())` or `zap.String("err", err.Error())` to `zap.Error(err)`.
Leave similar expressions with other messages unchanged, for example,
`zap.String("last_error", lastErr.Error())` or `zap.String("reason", ctx.Err().Error())`.

This change was made by applying the following patch:
```diff
@@
var err expression
@@
-zap.String("error", err.Error())
+zap.Error(err)

@@
var err expression
@@
-zap.String("err", err.Error())
+zap.Error(err)
```

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/local_object_storage/blobstor/exists.go

@@ -75,7 +75,7 @@ func (b *BlobStor) Exists(ctx context.Context, prm common.ExistsPrm) (common.Exi
 	for _, err := range errors[:len(errors)-1] {
 		b.log.Warn(ctx, logs.BlobstorErrorOccurredDuringObjectExistenceChecking,
 			zap.Stringer("address", prm.Address),
-			zap.String("error", err.Error()),
+			zap.Error(err),
 			zap.String("trace_id", tracingPkg.GetTraceID(ctx)))
 	}